ASCRS report: customized ablation, hyperopia and more

Manufacturers are addressing wavefront and LASIK issues. Laser and LASEK advances and “bladeless” flapmaking were discussed.

Issue: August 1, 2002

ByIrving J. Arons

PHILADELPHIA — Much has happened since I last addressed the subject of customized ablations in the January issues of Ocular Surgery News. More thought has been given to the inherent problems — some of which are now starting to be addressed — and more data have been collected by several companies involved in wavefront-guided custom ablation. As I wrote in January, results with “classic” LASIK have gotten much better thanks to improved lasers, trackers and keratomes. Most patients today can obtain uncorrected visual acuity close to or better than 20/20. But wavefront-guided ablations appears to offer even more — once the problems are solved.

Wavefront issues

In a recent article several ophthalmologists addressed some of the issues remaining to be solved. Ronald R. Krueger, MD, listed some of these: proper registration of wavefront information to the laser and its tracking system; wound healing issues — will the process of healing obliterate the lasers’ fine corrections?; biomechanical changes in the cornea, especially due to flapmaking in LASIK; the reliability and reproducibility of the wavefront information itself; and defining ablation patterns that truly correct wavefront patterns.

In ablating the cornea to correct aberrations using wavefront guidance, we are dealing with minute corrections. Such things as the angle at which the pulse strikes the cornea may make a difference in the effect. There are also hardware and software issues to be addressed. Dr. Krueger believes there is no single obstacle, but an array of challenges that need to be addressed one step at a time.

In the same article, Marguerite M. McDonald, MD, discussed the unpredictable biomechanical changes induced by the microkeratome cut that in turn alters the wavefront map in an unpredictable way. That is one reason driving the renewed interest in PRK and LASEK, and why some experts are discussing the possibility of a two-step LASIK procedure, in which first the flap is made and several weeks later the wavefront diagnostic is taken and the ablation performed.

Registration of wavefront data was discussed by Daniel S. Durrie, MD, (and Dr. McDonald in one of her ASCRS presentations). Dr. Durrie noted that the wavefront diagnostic was done on a seated patient, and then was attempted to be transferred to a supine patient some time later. He said it has been challenging just to center the ablation properly and to maintain orientation in the face of cyclotorsion from the patient’s change in posture. With custom ablation the challenge is to position the ablation perfectly, time after time.

With both the Alcon LADARVision system and the Bausch & Lomb Zyoptix with which he works, there is nothing to confirm that he is in the right place when starting the ablation. He would like to see technology that not only confirms the accuracy of the registration, but that actually performs the alignment and locks it into place. This is a quantum jump from current eye tracking, he noted.

“What’s needed for custom ablation is a failsafe device that will align the ablation pattern with the eye and not let the laser operate unless and until the two are perfectly registered,” he said.

Omar Hakim, MD, provided another note of caution. He discussed the changing basis of the wavefront, stating that it was dynamic, changing almost on a moment-to-moment basis. It changes with pupil size and accommodation, and evolves in other ways over longer periods of time as the eye matures and ages. Thus, a depiction at a point in time may not ideally depict what may happen at a later date.

“The wavefront we are looking at is the product of multiple variables that will differ over time. It is a static representation of a dynamic phenomenon,” he said. He suggests that using an adjustable corneal inlay in tandem with an adaptive technology, such as liquid crystal chemistry, might be able to create a dynamic wavefront correction.

LASIK issues

Richard L. Lindstrom, MD, addressed the biomechanical changes and wound healing processes that occur during LASIK. He said it might be difficult to correct the 1-µm-to-5-µm higher-order aberrations when the microkeratome induces 6 µm to 12 µm of aberration into each cornea in an unpredictable fashion. The final arbiter of corneal wound healing is the corneal epithelial cell, which is 6 µm to 8 µm thick.

“One epithelial cell in the wrong place induces more aberration than we are aiming to correct. [But] we should not be too pessimistic. This technology is still in its infancy,” he said.

Dr. McDonald, in her ASCRS presentation, noted that as the complexity of the correction increases, the requirements for centration, registration and tracking become more stringent. With just a 10° misalignment with cyclotorsion, a second-order cylinder correction results in a 30% residual error. The same misalignment for a sixth-order correction leaves a residual correction of 100%, with no impact on the desired correction.

“While centration, registration and tracking are important for all procedures, they become even more so for customized treatments, especially when complex higher-order corrections are attempted,” she concluded.

Manufacturers addressing problems

With all of that said, where are we today? Many of the problems noted here are being addressed by the laser manufacturers. Several have already addressed the problems of registration and cyclotorsion. Some are also beginning to address the problems induced by the flap with intraoperative pachymetry that accurately measures the real thickness of the flap and the depth of ablation in real time.

In his opening remarks at a Visx symposium on Wavefront Diagnostics, Outcomes and New Technologies held during the ASCRS meeting, Douglas D. Koch, MD, offered some perspective.

“In the 1990s, 20/20 was the minority outcome during the early days of PRK/LASIK. Today 20/20 outcomes are the majority, while in the future, with the advent of wavefront-guided ablations, 20/20 outcomes will be the minimum standard,” he said.

To show how some laser system manufacturers are doing, the table presents data available from some of the wavefront-guided custom ablation clinical trials. Some of the data collected to date show that 90% of patients or more (usually those with low-to-moderate myopia) can achieve UCVA of 20/20 or better. That is slightly better than is being achieved with “classic” nonwavefront-guided LASIK. And it promises to get better yet.

Innovations

Ophthalmic surgeons can look forward to some exciting innovations. Companies such as Carl Zeiss Meditec and WaveLight have begun incorporating tissue saving and prolate optimization algorithms into computer programs driving their lasers.

Asclepion introduced its CRS-Master product at ASCRS. This product combines topography and wavefront in an all-inclusive workstation with a new sophisticated software package that can be directly linked to the Meditec excimer laser. It allows the surgeon to integrate flap data, corneal thickness, pupil size, topography and wavefront data to define the ideal ablation profile for each eye. The program includes a tissue saving mode, and a prolate and night vision optimization mode, all built into its algorithms.

Nidek has a stand-alone combination aberrometer and topographer, the OPD-Scan.

Nidek, in its newest model of the EC-5000, the EC-5000CXII (to be introduced this fall), has added an improved, faster-responding four-beam infrared eye tracker, along with precision alignment and pupil registration of the undilated eye, a new cyclotorsion error correction algorithm and the multispot ablation (called MultiPoint) that was announced at last fall’s American Academy of Ophthalmology meeting, all combined with its combination wavefront and topographical diagnostic, the OPD-Scan, which can measure normal and highly aberrated eyes, which can measure normal and highly aberrated eyes.

Visx has also developed an iris registration cyclotorsion feature for their new tracker technology.

Schwind is among the first excimer laser producers to link online pachymetry using optical coherence tomography with their laser, to intraoperatively visualize the actual thickness of the flap (rather than the theoretical thickness), the depth of ablation and residual corneal thickness. This enables the surgeon to determine how much tissue has been ablated and how much residual stromal/corneal thickness remains.

The Sirius pachymeter, linked to the ESIRIS laser, allows real-time online visualization of the various stages of the LASIK treatment. Haag-Streit also produces a corneal pachymetry system, the HS Pachymeter, which can be used in a similar fashion to the Schwind Sirius pachymeter (which I believe comes from Zeiss Humphrey), for inline, real-time optical low-coherence reflectometry, to measure corneal thickness to a precision of 1 µm. Haag-Streit said it was working with at least two laser manufacturers who are evaluating its system (believed to be WaveLight and perhaps Visx).

Visx also announced just before the ASCRS meeting that it had signed an agreement with Tracey Technologies to market the Tracey Visual Function Analyzer (VFA) ray tracing device, along with the 20/10 Perfect Vision WaveScan aberrometer, to enable refractive surgeons to evaluate both normal eyes (with the WaveScan) and abnormal eyes (with the Tracey VFA), increasing the range of eyes that can be evaluated and treated.

Visx has also licensed the Ruiz multifocal patents for the treatment of presbyopia. Initial trial data on 23 eyes were reported at the meeting. The ablation pattern forms an aspheric cornea, giving good vision throughout the visual spectrum (near, intermediate and at distance).

Competition for “bladeless” flap

A new entrant, 20/10 Perfect Vision, showed its wares in the race for “bladeless” flap refractive surgery. The company displayed its new femtosecond laser in competition with IntraLase. Apparently, 20/10 Perfect Vision intends to couple its laser with its wavescan aberrometer (unlike the IntraLase). In the race for bladeless flaps, their laser is programmed to produce curved or contoured flaps (to preserve the natural shape of the cornea).

The femtosecond laser is capable of both stromal lenticle formation and removal and of intrastromal ablation, with cutting open the stroma. Because of gas formation within the stroma, the refractive effect may be limited to custom treatments of about 2 D of correction. For higher corrections, it may require a separate treatment sometime after the first treatment has healed. Intrastromal corrections are expected to be done for myopia, hyperopia and for presbyopia (via monovision).

LASEK advances

In another attempt to alleviate the effects of the microkeratome-created flap, several new laser epithelial keratomileusis techniques were discussed at the meeting. These were in addition to Dr. McDonald’s gel-assisted LASEK technique (see my AAO roundup in the January 1 issue).

The new techniques to preserve the epithelial tissue included hydrodissection (Richard C. Rashid, MD) with either saline or the use of GenTeal (or even GenTeal Gel in some cases), LASEK viscodissection (David W. Langerman, MD) and Ioannis G. Pallikaris, MD’s new subepithelial separator device, the SES. This is an automated microkeratome-based device for use with his subepithelial LASIK procedure (or EpiLASIK, as others have called it). The device, along with a suction ring, creates an epithelial flap without the need for alcohol, which allegedly results in faster healing and less pain for the patient. The SES is not currently available in the United States, but is expected to be on the international market by CIBA Vision in early 2003. Dr. Durrie has even suggested a new name for LASEK, to avoid its confusion with LASIK. He advocates calling LASEK procedures “advanced surface ablation.”

New hyperopia treatment (and beyond)

With its recent food and Drug Administration marketing approval, Refractec held a symposium to introduce conductive keratoplasty (CK). At the symposium, held in conjunction with Ophthalmology Management, Dr. Durrie, Robert K. Maloney, MD, and Dr. McDonald provided some very interesting information about the use of CK to treat astigmatism (Durrie), presbyopia (Maloney) and hyperopia — the U.S. clinical trial results (McDonald).

Dr. Durrie discussed his experience using CK for treating astigmatism. He found he could actually adjust the amount of correction intraoperatively by viewing the reflected ring formed by the cornea as the probe was applied in different quadrants.

Dr. Maloney discussed how the ViewPoint device could be used to provide near vision via monovision to correct for presbyopia. With the patients he has treated, the effect looked good out to 3 years. Although the device had only been approved for hyperopia without astigmatism, it is being used successfully off-label for treating astigmatism, presbyopia and over- and undercorrected LASIK. He noted there was a very fast learning curve and was foolproof to use. In his presbyopia trials, after 9 months, 79% of his patients were seeing 20/20 J2; 83% were 20/25 J3; 87% were 20/32 J5; and 91% were 20/40 J6. Stability was achieved in within 2 to 3 weeks.

Dr. Maloney also described how the heating effect of CK was different from what was achieved with laser thermal keratoplasty (using the Sunrise laser). In a thermal effects study done by A.J. Welch’s group at the University of Texas, the heating zone profile produced by the ViewPoint needle probe was more uniform, thinner and deeper (a column of heating 250 µm wide and 80% of the cornea deep), while the LTK laser pulses produced a much wider heating zone that concentrated in the upper stroma. That might explain the anticipated less regression with the CK device compared to the LTK. When Dr. McDonald was asked about the “temporary” label given to Refractec by the FDA, she said it was because of their previous experience, and it would probably be removed when Refractec provided its 2-year data to the agency.

Dr. McDonald discussed the FDA trial data that led to the 0.75 D to +3.00 D of hyperopia approval. At 12 months, 54% of patients achieved 20/20; 74% were 20/25; and 91% were 20/40 or better. As for stability, there was a 0.9 D loss in the first 3 to 6 months, 0.03 D in the 9-to-12 month period, and only 0.04 D during months 12 to 24.

There is, however, one caveat to consider about the use of ViewPoint CK for treating hyperopia. Most low hyperopes are comfortable with their vision. It is only when they become presbyopic that they begin looking for alternatives. Thus, the real problem to be solved with CK is presbyopia, not low-to-moderate hyperopia, along with some degree of astigmatism (and over/undercorrected LASIK). But because of the device’s low price, ease of its use and its safety record, once the “temporary” label is removed, I would expect to see widespread use of this device as an alternative to hyperopic LASIK.

For Your Information:
Wavefront-guided ablation: what are the obstacles to implementation? Refractive Eyecare for Ophthalmologists. April 2002.
McDonald MM. Importance of tracking, centration and registration in custom ablations. Presented at the American Society of Cataract and Refractive Surgery annual meeting, Philadelphia, June 2002.
Irving J. Arons is managing director of Spectrum Consulting with offices at 4 Harvard St., Peabody, MA 01960; phone and fax: (978) 531-0939; e-mail: iarons@erols.com. Mr. Arons has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.